1. Field of the Invention
This invention relates to lubricating oils for refrigeration equipment, and more particularly to such lubricating oil compositions which are suitable for use in refrigerant compressors of a rotary design.
2. Prior Art
Numerous refrigeration lubricating oils have been proposed; however, such oils have advantages and disadvantages when taking into account the following typical properties required in service:
(1) Optimum viscosity.
(2) Good low temperature characteristics including low pour point and low floc point.
(3) Low critical temperature for dissolution in refrigerants.
(4) Good data on sealed-tube tests; that is, no discoloration, no precipitation and no copper plating.
(5) Good thermal and chemical stability even in the presence of halogen-containing refrigerants.
In addition to the foregoing criteria for the value of refrigeration lubricants, there is another important factor which must be considered, particularly where the lubricating oil is intended to be used in refrigerators operating on a compressor system. It is the capability of a given oil to establish a gas seal between the low pressure side (suction) and the high pressure side (discharge). Modern refrigeration compressors have a tendency to encounter increased gas temperatures at the discharge side, which would lead to reduced oil viscosity and hence to loss in the sealability.
Compressor-type refrigerators are structurally classified into ones of a reciprocating, centrifugal and rotary type. For its compactness, high performance and quite service, the rotary-type compressor finds wide application in the areas of home-use refrigerators, coolers, automotive air-conditioning equipment and the like.
Refrigeration lubricating oils are also considered satisfactory if the oil mists entrained with high pressure discharge gas in the compressor system are readily separated from the refrigerant and if such entrained oil is easily returned from condensers or evaporators. Such oil behaviors largely depend upon the viscosity.
Refrigerator oils are required to more strictly meet with the aforesaid various properties and capabilities particularly where they are used in rotary compressors suitable for high speed operation.
Traditionally used as refrigerator lubricating oils, are naphthenic mineral oils, low pour point paraffinic mineral oils, heavy alkylbenzenes and the like which have a kinetic viscosity of 20-150 cSt at a temperature of 37.8.degree. C., or mixtures thereof.
These starting oils often incorporate certain additives. Typical examples of such lubricating oils are disclosed for instance in Japanese Patent Publication Nos. 11940/65, 4107/74, 13483/74, 13829/74, 19084/74, 37647/76, 45013/76, 39509/77, 43722/77 and 17602/78 and Japanese Laid-Open Patent Application Nos. 4532/71, 606/73, 47498/74, 22971/76, 28503/77 and 54707/77.
It is also known that polyglycols are a good material for lubrication of refrigeration equipment. For example, K. S. Sanvordenker et al in the ASHRAE Symposium Nasso, June 29, 1972 indicated that polyglycol diethers of a 100 SUS grade having a viscosity of 102 SUS (21.0 cSt) at a temperature of 37.8.degree. C. and viscosity of 44 SUS (5.3 cSt) at a temperature of 98.9.degree. C., as well as polyglycol monoethers of a 165 SUS grade may be used as a refrigerator oil. J. M. Russ et al in the Lubrication Engineering Vol. 2,151 also introduced polyglycol oils tradenamed "UCON" having a viscosity of 5.9-22.0 cSt at a temperature of 98.9.degree. C. and a viscosity index of 140-147.
None of the above listed prior lubricants has been found satisfactory for the achievement of the objects of the present invention which will appear hereafter.